Optical filters play an important role in wavelength division multiplexing (WDM) communication systems. WDM systems achieve high bandwidth transmission by combining multiple optical channels, each of a different wavelength range, in an optical fiber. A filter is utilized to extract a specific optical channel from a multi-channel signal at a receiver side, and can be either fixed to a given wavelength range or tunable across a range of wavelengths.
Integrated optics provides for a compact method to realize an optical filter, and especially a tunable optical filter. One method of realizing filters in integrated optics technology is to combine multiple optical resonators [B. E. Little et al, “Microning Resonator Channel Dropping Filters”, IEEE J. Lightwave Tech. 15, 998–1005 (1997)].
Generally, a tunable filter is characterized by such key parameters as bandwidth, insertion loss, attenuation (rejection) of out of band signal, free spectral range (FSR), and turning range.
An important feature, characterizing all optical resonators and resonator based devices, is the periodicity of their spectral response, i.e., the spectral response repeats itself with a period known as the Free Spectral Range (FSR). FIG. 1A illustrates the spectral response (transfer function) of a resonator coupled to input and output ports The FSR of such a device is the spectral spacing between the peaks of the transfer function. In optics, such a device can be realized, for example by a Fabry-Perot (FP) resonator comprised of a pair of partially reflecting mirrors (FIG. 1B), or by a ring resonator coupled to two waveguides which serve as input/output ports (FIG. 1C). The geometrical structure and constituent materials of the resonator device determine the total roundtrip delay of the device, which is the inverse of the FSR.
A resonator is characterized by such parameters as FSR, loss per roundtrip and coupling to input/output ports. The FSR indicates the spectral period of the resonator, and the coupling indicates the fraction of the light intensity in the input/output ports that is coupled into the resonator (and vice versa). All these parameters affect the filter profile. For example, a filter bandwidth can be narrowed by (1) decreasing the coupling, or (2) by decreasing the FSR (increasing the resonator roundtrip) and keeping tie coupling level constant. Decreasing the coupling also results in an increase of the out of band signal attenuation and the input to filtered output ratio (insertion loss) of the filter.
Generally, the requirements for filters in optical communications involve a narrow bandwidth and a wide FSR. Therefore, the known resonator based filters (e.g., WO 00/72065) were designed accordingly (i.e., large FSR and small coupling in order to achieve narrow bandwidth). In principal, this design approach exhibits superior filter performance. However, when accounting for the resonators' loss per roundtrip, the situation becomes more complex since for ring resonators, a large FSR implies small radii, which in turn implies higher radiation related losses. Hence, it is clear that not every filter shape or FSR may be achieved within a given loss budget.